Various internal combustion engines have been proposed which can switch over the operating properties of the intake valves or exhaust valves according to the speed and load conditions of the engine. As well known, such a switch over would produce a substantial shock which can be perceived by the vehicle occupant. A valve operating condition switch over device has already been put into practical use to achieve a switch over of the valve operating condition without causing a large change in the engine output in a smooth fashion by switching over the valve operating condition in a step-wise manner at a point where the engine output obtained by the valve operating condition for low speed condition coincides with the engine output obtained by the valve operating condition for high speed condition (Japanese patent laid open publication No. 02-42105).
A traction control device is also put into practical use for automatically controlling the traction force of the driven wheels by detecting the slip ratio of the driven wheels from the speed deviation between the driven wheels and the undriven wheels, and reducing the engine output through the use of a leaner air/fuel ratio when an excessive slipping of the driven wheels is detected (Reference should be made to Japanese patent laid open publication No. 02-157440 and others).
Copending US patent application based on Japanese patent application No. 03-180553 discloses a variable valve actuation control device comprising means for eliminating the shocks resulting from the switch over of the valve operating condition by prohibiting the switch over under certain conditions. The disclosure of this patent application is incorporated into the present application by reference.
In mass produced internal combustion engines, it is not possible to totally eliminate some unevenness in their specifications due to the variations in the manufacturing and control precisions in manufacturing injectors and camshafts. In particular, the error in the air fuel ratio during a traction control involving a leaner air fuel ratio results in a relatively large change in the actual output torque (refer to .DELTA.TR&lt;.DELTA.TL). Therefore, if the valve operating mode is switched over in a normal manner during a traction control, the actual outputs obtained in the high speed mode and the low speed mode may not agree with each other, and it may happen that the low speed mode gives rise to a lower output torque than the high speed mode. In such a case, the switch over of the valve operating mode would rather lower the output torque, thereby reducing the slip ratio of the driven wheels, with the result that the traction control is discontinued, and the normal air fuel ratio is restored. This in turn would increase the output torque, thereby increasing the slip ratio, and the traction control is thereby restored in such a manner that a hunting phenomenon may be produced.